Method of making electrical resistances



J y 93 R. c. BENNER ET AL 1,814,583

METHOD OF MAKING ELECTRICAL RESISTANCES Filed April 5. 1927 INVENTORS(222 @M Patented July 14, 1931 I UNITED STATES PATENT OFFICE RAYMONDBENNER, OF NIAGARA FALLS, AND EVERETT S. CAPRON, OF BUFFALO, NEW YORK,ASSIGNORS TO THE CARBORUNDUM COMPANY, OF NIAGARA FALLS, NEW YORK, ACORPORATION OF PENNSYLVANIA METHOD OF MAKING ELECTRICAL RESISTANCESApplication filed April 5, 1927. Serial No. 181,078.

This invention relates to electrical resistance devices, and moreparticularly extremely high resistances of the type commonly used inradio receiving circuits for grid leaks, couplings for resistanceamplification, and the like, and to the method of making such devices.

According to the present invention, it is proposed to provide aresistance which is non-inductive, non-microphoniqwhich does not absorbmoisture from the air, and whose resistance remains constant and isunaffected by changes in atmospheric humidity.

A further important feature of the present invention is the method ofmanufacturing the resistance so as to render the same non-microphonicand immune to atmospheric conditions, as well as a simple process forvarying the resistance during the process of manufacture as may benecessary in a convenient and simple manner.

Our invention is more especially applicable to the class of resistancesdescribed in United States patent to Hutchins et al,

7 No. 1,322,573, dated November 25, 1919,

which consists, briefly, of fabricating resistors from a plastic mixtureof silicon carbide and an easily fusible clay containing relativelylarge amounts of iron and calcining theresistors at a temperature atwhich part of the iron oxide is reduced by the silicon carbide tometallic iron.

In the accompanying drawings we have illustrated certainsteps in theprocess of manufacturing a resistance according to the presentinvention. In'the drawings:

Figure 1 represents a length of crude resistance rod; I

Figure 2 shows the ends of the rod bevelled 40 to receive-the. metalliccap;

Figure 3 shows the rod with a metallic coating applied to the endsthereof; and

Figure 4 shows the assembled resistance device, one end of the devicebeing shownin section. In the practice of the present invention, thematerial from which the rod is formed is mixed, as described in the saidUnited States patent to Hutchins et al, molded to the desired shape, andbaked at the desired temperature, as described in said patent, thebaking and composition of'the mixture being so regulated that theresistance of the molded rods is higher than that required of thecompleted resistor. \Vhen the rod has been baked and is removed from thefurnace, it is thoroughly hard and is of the shape shown in Figure 1. InFigure 1 the rod of resistance material is designated 2.

In order to accommodate the ends of the rod to metallic caps of the typecommonly employed in devices of this kind, the ends of the baked rodsare bevelled either before or after baking, preferably before, onaccount of the greater ease of performing the operation, asindicated at3 in Figure 2. This may be done by holding the rod against an abrasivewheel at the proper angle.

The bevelled ends 3 of the rod 2 are then coated with a soft metal, suchas lead, or lead alloy, by holding the end of the rod against a wheelmade of lead, lead alloy, or other suitable soft metal. This leadcoating, designated 4, forms a metallic contact material to the ends ofthe rod which is not microphonic.

Metal end caps 5 are then forced on to the coated ends 4 of the rod. Thelead in the joint between themetallic caps and the composition rod formsa good conducting path, L

which is not microphonic, whereas graphite sodium silicate materialheretofore com-. monly used in devices of this nature is microphonic attimes.

After the devices have been assembled, as shown in Figure 4, they areimmersed in a molten bath of a suitable non-conducting wax, pitch orresin which does not absorb water readily and which has a high permanentelectrical resistance or dielectric constant, such for instance, asparaflin, ceresin or .Superla wax.

If it is desired to simply waterproof the rod, it is immersed in themolten paraffin at 250 F. for about four minutes. This does notappreciably change the resistance of the device, but if it is desired todecrease the resistance, the device is left in the hot paraffinuntilsuch time as the resistance is reduced to the required value.Numerous experiments have shown that the resistance of the rods can bedecreased at a rape of approximately one. per cent per hour by immersionin the molten parafiin at 250 F.

After the resistance devices have been immersed in the parafiin for thedesired length of time, they are taken out of the bath and the excessparaffin is removed centrifugally or by wiping or by any other suitablemethod, and while the resistors are still hot,

i. e. still above the melting point of the paraflin. After the resistorshave cooled, they are classified by any suitable measuring apparatus. c

It is obvious-that by controlling the composition of the rods, that is,the relative proportions of clay and silicon carbide, the length of timeand the temperature of calcining, as well as the length of time ofheating in paraffin, it is possible to make resistance rods varying fromover 10 megohms to about 1,000 ohms without varying the size of the unitbeyond that shown in the accompanying drawings Resistance units madeinthis manner are non-inductive, permanently non-microphonic, and areunafiected by atmospheric humidity.

While we have illustrated our invention particularly inconnection withresistances of the familiar cartridge type, it will be understood thatthe invention is not limited to the particular. form and style ofresistance illustrated, and that various modifications and changes inthe construction of the device may be made within the contemplation ofour invention and'under the scope of the following claims.

We claim:

1. In the manufacture of rigid resistor rods comprised principally ofsilicon carbide, the step of heat treating the rod in a bath of moltenparaffin to vary the conductivity thereof.

2. Inthe manufacture of resistor rods, the steps comprising forming arigid rod comprised principally of silicon carbide, heat treating therod 1n the presence of parafiin for a predetermined time, and thereafterremoving the excess paraffin.

3. In the manufacture of ceramically bonded resistor rods, the stepwhich comprises immersing the rods in a bath of paraffin and varying theperiod of immersion to vary the conductivity of the rod.

4. The step in the manufacture of silicon carbide resistors whichcomprises immersing the resistors in a hot bath of parafiin and varyingthe period ofimmersion to vary the conductivity of the rods.

5. In the manufacture of silicon carbide I comprised principallyofsilicon carbide, the

step of impregnating the perviously formed and matured rods with acompound such as paraflin capable of decreasing the resistance of the'rods and possessing a high dielectric constant.

7. In the manufacture of rigid self-supporting resistor elementscontaining a clay binder having iron oxide, the steps which compriseforming the resistor element from the raw mix, firing the element tomature the binder and reduce the iron oxide, and thereafter immersingthe formed rigid rod in a bath of molten paraffin for a time longer thanten minutes, thereby ad usting its resistance value.

In testimony whereof we have hereunto

